The invention relates to a method of process monitoring and quality control for laser welded butt joints, especially in thin sheet metal. The invention also relates to an apparatus for carrying out the method. Characteristic process-sensitive data are obtained by means of pyrometric temperature measurements and are evaluated for quality control. By setting tolerance limits, the quality of the weld can be assured within a predetermined framework.
In recent times, the possibility of using a temperature value immediately behind the liquid-solid interface of the weld for the purpose of process monitoring has been recognized, and a relationship has been established between the measured temperature value and process parameters such as laser power output or working gas throughput (U.S. Pat. No. 4,663,513). The drawback of this method is that important parameters have to be assumed constant and changes in these parameters greatly distort the measurements obtained. For example, if there is just a slight dislocation of the beam, it is no longer possible to obtain a clear correlation between the measured values and the process parameters. Since, in principle, the behaviour of only one parameter can be evaluated by this method in relation to the change in temperature caused by that parameter, the method cannot be used as documentable evidence of weld quality.
For laser welding, coupled pyrometric measurements have hitherto been used only for research and parameter optimization. A known application for production utilizes an array of pyrodetectors which are all trained on the point of impingement of the laser and whose signals are electronically averaged in order to generate more reliable mean surface temperatures, particularly where relatively large surface areas are being treated (EP 0562492).
In modern industrial processes for the production of laser welded panels for motor vehicles, "tailored blanks", that is to say pre-cut flat sheets with dissimilar thicknesses and material properties, are butt welded together. In the cooling zone after the liquid-solid interface, a temperature distribution develops which is normally asymmetrical about the weld centreline and which is typical for the rate of travel, energy input, and pairing of materials. Evaluating the temperature recorded at only one spot does not afford a clear correlation with process parameters.
In principle, thermographic image analyses provide a solution to the problem. Image processing can greatly simplify and shorten the operations of calibration and setting of the pyrometer arrangement for new combinations of materials. By filtering the data obtained from thermograms, the described pyrometer arrangement can be fully simulated and signals identical to those obtained with this arrangement can be fed to the connected instrument chain for evaluation. Also, by using known algorithms, further inferences can be drawn as to weld quality from the additional information obtained. Production and filing of reference thermograms is advantageous here, and the build-up of self-learning systems is an obvious choice.
However, the use of this method on-line requires undesirably longocomputing times and is over-elaborate since only a small part of the information obtained is actually needed for process monitoring. Another obstacle is the high cost of commercial implementation.
Hence it is the fundamental object of the invention to provide a method whereby several relevant process parameters can be recorded in a manner which is simple and suitable for industrial application.
It has been found that by taking temperature measurements at at least two defined points it is possible to establish the link with the process parameters and to make the process transparent. This is done by aiming preferably high-speed pyrodetectors at defined points on either side of the seam immediately behind the liquid-solid interface of the weld metal. An individual measurement spot will typically contain an area of approx. 1 mm.sup.2.
The signals from the individual pyrometers are linked together by electronic signal-processing. Logical operations such as addition, subtraction or division are available as standard routines.